git: ignoring a file which *is* version controlled

Question

A .gitignore file allows to ignore files from version controlling them at all.

We have a different situation: we want to place in the repository some config

Answer 1

I define the following aliases in my .gitconfig file:

[alias]
  ignore = update-index --assume-unchanged
  unignore = update-index --no-assume-unchanged

That should do exactly what you want: running git ignore some-file will treat the file as unchanged no matter what you do to it, until you run git unignore some-file.